It may have only lasted seven minutes, there may have been no time for questions from the media, and it may have contained no major new policy commitments, but in breaking his near two-year silence on environmental issues David Cameron still made an important intervention that should provide a boost to the UK's fast-expanding green economy.

This was an eloquent and unequivocal statement of support for the low carbon economy in general and the renewables sector in particular. It may fall short of what green campaigners had hoped for, but the comments still mark an important intervention and green businesses and investors should be heartened by the Prime Minister's remarks.

His assertion that "I passionately believe the rapid growth of renewables is vital to our future" was not only a none too subtle rebuke to those figures within his own party who want to see a rolling back of renewable energy, but a clear commitment to the continued growth of the industry.

Moreover, he demonstrated an awareness of the economic and commercial success enjoyed by the green economy, announcing a series of new investments and declaring that "renewable energy is not just good for our environment but we believe it's very good business, too".

Yes, he acknowledged concerns over the cost of renewable energy, noting that "we need greener energy, but we need cheaper energy, too". But he also made plain that he understood that the cost of many forms of renewables was falling rapidly, stating that renewables could be "among the cheapest" sources of energy in a few short years. This government will continue to cut renewable energy subsidies as costs fall, but the Prime Minister is now on record as saying he does not want to see the renewable energy industry falter.

Perhaps most important of all, Cameron pledged to deliver a clearer and more stable policy environment in the future. Unsurprisingly, he did not raise the solar feed-in tariff fiasco or the chilling effect that the Chancellor's anti-green comments have had on investor confidence, but he did assert that investors could be assured of more clarity in the future. "When we have made a commitment to a project we will honour it in full," he said – too little, too late for those solar installers currently out of work, but music to the ears of prospective investors.

Speaking to Energy and Climate Change Secretary Ed Davey following Cameron's speech, he told me the watchword for green policies going forward was "predictability". "Stability" would imply that things would never change, he said, which is not the case in a sector where technologies are evolving so quickly. But he insisted all the government's current reforms are designed to give businesses the kind of "predictability" they need to move forward with investments.

Of course, none of this is to suggest that the Prime Minister's short intervention resolved the challenges faced by the green economy, nor did it compensate for the damage done by recent policy mis-steps. There remains plenty for green campaigners and businesses to be frustrated, even angry about.

The downgrading of the speech from keynote address to short opening remarks was a slight, whichever way you look at it, while the absence of any new policy commitments or truly large-scale investment announcements was hugely disappointing.

Cameron may have won praise from some of the energy ministers in the audience (remember the UK policy environment really is progressive compared to most countries), but he singularly failed to acknowledge the damage done by the mishandling of cuts to solar incentives and the continued uncertainty over the Green Deal and several other policies.

Similarly, critics can justifiably claim that the speech focused on renewable energy because this is one of the few green areas where the government has enjoyed some success. There was a notable silence on biodiversity, resource efficiency, waste, nuclear, and a raft of other areas where progress has been less pronounced.

Meanwhile, the Prime Minister's green claims are badly undermined by the government's continued support for shale gas and other forms of fossil fuels, not to mention airport expansion and road-building. Industrialised nations can never expect to win the full endorsement of green business leaders as long as their response to climate change remains well short of what scientists recommend.

Most importantly, there was again a reluctance to marry the fast-expanding green economy to the broader growth plan the government so desperately needs. The speech failed to invoke any real sense of passion or urgency, failing once again to adequately express the risks associated with climate change or the immense growth opportunity provided by green businesses. For a summit dedicated to exciting and innovative clean technologies there was hardly any mention of the inspiring green technologies, many of them being developed in the UK, that promise to change the way we live for the better. I long for the day a minister gets up and speaks passionately about cutting-edge solar glass technologies, lightweight nano-materials, next-generation electric sports cars, or algae-based jet fuels – it is my bet these technologies and many others would interest audiences far more than they think.

These absences meant there was plenty of ammunition for green groups to criticise the Prime Minister for what he did not say, rather than praise him for what he did say.

WWF called the speech a "damp squib", Friends of the Earth said it fell "a long way short" of what was needed, while the normally measured Climate Group accused the Prime Minister of "a costly, short-sighted error of judgement" that saw him side "with those in his government that feel that the green agenda is a 'burden'."

This reaction is understandable, but it also feels reflexive and counter-productive when it comes on the same day as the Prime Minister has clearly pledged his support for clean energy. Yes, it would have been nice to see more policy detail, and it remains the job of campaigners to constantly call for more ambition. But Cameron and the green wing of the government deserve at least some qualified praise for reiterating his broadly progressive position on renewable energy and the environment.

If rhetoric is important when it undermines green investor confidence, as was the case with the Chancellor's comments last autumn, then it is also important when it aims to boost investor confidence.

The Prime Minister has today effectively hitched his wagon to the renewable energy industry and offered clear and explicit support for its continued growth. He will almost certainly make decisions in the future that the industry disagrees with, but he cannot now perform a U-turn on major renewables policies without shredding his credibility.

The green business sector still needs to work out how to co-operate with the wider business community to combat the anti-green Tea Partification of the right wing of the Conservative Party. It is increasingly clear that unless green business leaders get better at making their case a future majority Conservative government led by George Osborne or someone else from the right is likely to offer scant regard for the low carbon economy.

But for now Cameron has effectively told his backbenchers that he is not going to abandon the green agenda and that he sees the growth of the renewables sector and the continued development of the "greenest government ever" as "vital" to the UK's interests. He has taken a small, but important step towards restoring the green investor confidence that was starting to waver. Close attention will now be paid to the policy detail due to be released over the next few months to see if he lives up to his promises.

The speech may have only lasted seven minutes, but its implications are likely to be felt for much, much longer.